DESCRIPTION: Sharing of syringes and other injection equipment among people who inject drugs (PWID) is a significant risk factor for transmission and acquisition of blood-borne diseases including HIV and hepatitis C. A recent study of young PWID in the Chicago metropolitan area found that over 40% used a shared syringe, and over 75% shared other injection equipment in the past six months. This study also found that substance- induced major depression (SIMD) and borderline personality disorder (BPD) were significantly associated with receptive syringe sharing. Emotion dysregulation is a hallmark symptom of BPD, and is implicated in major depressive disorders. These findings point to the possible role of negative affect and emotion regulation difficulties in perpetuating injection risk behavior. Prevention intervention programs for PWID may be able to help address these problems, however further research is needed to elucidate the mechanisms that drive the linkages between negative affect, emotion dysregulation and HIV/HCV risk behavior among young PWID. In order to more fully understand the relationship between emotion dysregulation and HIV/HCV risk behavior, we need to collect data in real-time, in the context of everyday activities. Ecological momentary assessment (EMA) is a method for collecting such data, commonly using a portable electronic device such as a handheld computer or mobile phone. This methodology greatly reduces biases associated with retrospective recall, and can capture the interplay of mood and behavior over time. We will use a stratified random time-sampling design to obtain a representative sample of PWID's emotional state, substance use, and injection behavior over a 2-week period. Baseline assessments will be conducted to evaluate mental health and emotion dysregulation. This study will test the acceptability and feasibility of ecological momentary assessment (EMA) to study mood and behavior in young people who inject drugs (PWID), and will gather preliminary data on emotion regulation, mood, and injection behavior among young PWID in the context of everyday activities. The results of this study will inform the development of a larger study to examine the associations among negative affect, emotion dysregulation, and injection risk behavior in PWID. This research will have implications for intervention practices for PWID with emotion regulation difficulties, to reduce risky injection behavior and to promote entry into treatment for substance use and mental health.